Screen wheel filters as in for example DE 103 58 672 [U.S. Pat. No. 8,202,425] are known in the prior art. The known screen wheel filter comprises a filter-element changing station. DE 102 25 601 [U.S. Pat. No. 6,843,915] discloses a screen wheel filter that is also equipped with a backflushing station.
Screen wheel filters serve to purify a high-viscosity melt, such as from an extruder, and to feed it into a further process stage, for example an injection-molding machine, granulator or blow molder. In such screen wheel filters, the screen wheels are rotated in steps substantially dependent on degree of contamination, and the rotation, as is known from DE 199 61 426 [US 2003/0132146] or DE 103 26 487 [2006/0157879], occurs at specific time intervals in which a corresponding degree of contamination of the screen disk is expected, or occurs depending on a differential pressure signal that indicates the degree of contamination of the screen disk. Periodic, large stationary periods thus exist between the rotation steps. Together, the rotating and stationary periods are routinely so large that the melt remaining in the screen inserts experiences a substantial thermal decomposition or oxidative decomposition such that before each re-entry of a screen insert into the melt passage, for example into the changing station according to DE 103 58 672, the screen insert must be removed from the screen wheel and changed for a new screen insert, or must otherwise be backflushed and thus cleaned before entry into the melt passage, according to DE 102 25 601.
Substantial thermal decomposition or oxidative decomposition is to be here understood, for example, as a heat reduction of the molecular chain length of polymers that is so great that material that has been so altered can no longer be fed through the production flow without degrading the final product. A substantial thermal decomposition or oxidative decomposition is expected if more than 5% of the molecular chains of the respective polymer have already been split. A substantial thermal decomposition or oxidative decomposition also occurs if the main component of the plastic or the additives contained in the plastic are so greatly decomposed that the surface of the processing system is affected by the products of decomposition.
Here, the times in which a substantial thermal decomposition or oxidative decomposition occurs are dependent on material and temperature. Thermally stable polymers can be held at processing temperature for up to 60 min without the occurrence of substantial thermal decomposition or oxidative decomposition, while in thermally sensitive melts, substantial thermal decomposition or oxidative decomposition already occurs in <5 min.
In the exchange of a screen insert, the screen insert is removed from the screen wheel with the melt remaining in the screen insert, and exchanged for a new screen insert that does not yet hold any melt. The melt remaining in the removed screen insert is thereby lost to the production process.
However also during backflushing, material is removed from the already purified melt stream and pressed against the screen insert in the backflush slot. Here, the melt located in the screen inserts is exchanged for new melt. The original melt remaining in the screen inserts is disposed of on disposal of in the ejector of the screen filter. Here too, large losses of melt occur, whereby both a changing of the screen insert and a backflushing of the screen insert increase the expense of operation of the screen wheel filter.
Often, the changing of the screen insert and/or the backflushing of the screen insert takes place even though it is not contaminated, merely to prevent thermal or oxidative decomposition.